1. FIELD OF THE INVENTION
This invention relates to improvements in a control system for induction motor driven electric car.
2. DESCRIPTION OF THE PRIOR ART
An induction motor driven electric car is equipped with an inverter receiving direct current and converting it into three-phase alternating current of variable voltage and variable frequency, and a plurality of three-phase induction motors fed from the inverter to drive the electric car.
Typically, an output (operating) frequency f.sub.INV of the inverter is set by adding or subtracting a slip frequency fs to and from a frequency fr corresponding to a rotation speed of the induction motor (equivalent to electric car speed). The inverter is applied with a command proportional to the inverter frequency command f.sub.INV to set its output voltage V.
As regards the slip frequency fs, a fundamental command fsp in compliance with required torque is preset and this command is corrected by an output signal of a current control system. A current command applied to the current control system is made constant at least within a variable voltage and variable frequency (VVVF) region of the inverter and has two modes to be described later within a constant voltage and variable frequency (CVVF) region. In contrast to the current command, a motor current value to be fed back takes the form of a mean value of a plurality of motor currents or a maximum value of a plurality of motor currents.
Especially where re-adhering performance is considered significantly, a maximum current control system is desirably used. In this system, a maximum current is detected and therefore, even when part of driving wheels slip and the current flowing to a motor connected to the slipping shaft decreases, the partial slipping does not affect the other wheels. It follows therefore that the slipping shaft can be again brought into the adhering status without unnecessarily increasing the current to increase torque. Such a control system is disclosed in, for example, "Development of PWM Control System for Electric Locomotive", 18th Domestic Symposium Collected Papers No. 423 on Utilization of Cybernetics in Railway (November, 1981), Japan Railway Cybernetics Conference, pp. 245-249, especially, FIG. 6 and its description.
In an induction motor driven electric car equipped with the known maximum current control system, however, it happens that a desired level of torque can not be obtained.